It depends; living in Beijing can really help standardize your accent; but you won't benefit if you mainly hang around with other foreigners. It also doesn't help that most decent ladies that you'll meet often speak very fluent English. Go to underground rock bars with few foreigners about, make local friends and avoid most of the glitzy, flashy stuff and you'll actually have a decent Chinese learning experience. It's far better than Shanghai at least.

Beijing actually has universities with not so bad standards of teaching; but living there is a nightmare unless your company is paying most of your expenses. Even then, it's polluted, crowded and not as nice as it used to be.